Determine Total Radiated Power Performance Of PAs

TOTAL RADIATED POWER (TRP) is an increasingly important performance parameter for power amplifiers (PAs) in Global System for Mobile Communications (GSM) infrastructure equipment. A PA's TRP performance is very much affected by varying load impedance, which can be measured by simulating an over-the-air (OTA) testing environment. The Cellular Telecommunications Industry Association (CTIA) has an advanced specification for this topic, dubbed revision 2. 1 of the Test Plan for Mobile Station Over the Air Performance. In a four-page application note, RFMD (www.rfmd.com) examines this testing approach for TRP measurements.

The note, which is titled, "Total Radiated Power (TRP) Implications for a GSM Power Amplifier," provides an overview of OTA testing and TRP. In simulated-use mode, OTA testing involves mounting a mobile device on a Specific Anthropomorphic Mannequin (SAM) phantom head. The mobile device also is rotated through various spherical angles with respect to the measurement antenna. The antenna impedance varies because it is affected by the SAM phantom head as well as the angle of rotation, presenting a varying load impedance to the PA.

The note details a test setup that has been generated to evaluate the TRP performance of the PA. In order to pass TRP requirements, the PA must provide enough output power to overcome mismatch loss. Because TRP performance is impacted by the antenna and the PA, the PA must be include compensation circuitry that counteracts the effects of load-impedance variation presented by the antenna.